MPs are to vote on Thursday on a 48-hour extension to the deadline to register for the EU referendum, which is expected to allow tens of thousands more people to vote in the June 23 poll.

The emergency legislation came after the Government registration website crashed on Tuesday evening as almost a quarter of a million people tried to apply in the final hours before the midnight deadline.

Prime Minister David Cameron urged would-be voters to keep on submitting their details, saying he was working "urgently" to ensure they would be able to take part in the referendum.

But senior pro-Brexit Tory Bernard Jenkin, the chair of the Commons Public Administration and Constitutional Affairs Committee, warned that the extension would be vulnerable to judicial review.

He told MPs that any attempt to rewrite the rules in a substantial way "would be complete madness and make this country look like an absolute shambles".

Campaigners for EU withdrawal were branded "a bunch of conspiracy theorists" by a Cabinet minister after the chief executive of Vote Leave accused the Government of using the deadline extension to try to "skew" the result of the EU referendum by extending the deadline.

In a message to supporters, Matthew Elliott said "we know the Government and their allies are trying to register as many likely Remain voters as possible", adding: "Don't let the Government skew the result of the referendum - make sure you and your friends are all registered today."

There was speculation that many of those blocked from joining the electoral roll by Tuesday's website meltdown may be younger voters, who polls suggest predominantly back EU membership.

But Downing Street said there was no way the Government could know the voting intentions of those applying to register.

Responding to Mr Elliott's comments, Education Secretary Nicky Morgan said: "Anyone who cares about British democracy, whichever side of the argument they are on, should support any attempts to get new voters registered to have their say in this referendum.

"Vote Leave's anger is a reflection of their paranoid, anti-democratic attitudes. They want to con the British people into voting to leave Europe on a derisory turnout. However much they talk about sovereignty, it is clear they have no respect for the democratic rights of the British people."

Following emergency discussions with the Electoral Commission and opposition parties, the Government plans to table a statutory instrument to amend the EU Referendum Conduct Regulations, reducing from five to three the number of working days before the poll that the electoral lists must be published.

This has the effect of extending the registration deadline to the end of Thursday, while preserving a separate five-day period for appeals against entries on the register. Mr Cameron's spokeswoman said ministers were confident this would be "legally watertight" against challenge.

Although the website was only down for two hours, it has been decided to extend the deadline by 48 hours in order to allow time for disappointed would-be voters to learn that they are being given a further chance to sign up, said Downing Street, which confirmed that the Vote Leave and Stronger In campaigns were not involved in the discussions.

Downing Street said 214,000 people were trying to use the www.gov.uk/register-to-vote website between 9pm and 10pm on Tuesday, but it was not known how many were prevented by the system overload from registering before the midnight deadline.

Measures have been taken to try to ensure that the system is robust enough to deal with further high volumes of applications over the course of Thursday, said the PM's spokeswoman. Interest in the referendum may spike once more that evening, when ITV broadcasts a live EU debate featuring Boris Johnson.

Leading Brexit campaigner Michael Gove warned that changes to the voting regulations would take the Government into "complex legal waters", but added: "In my heart is a desire to ensure that everyone possible can be given the vote."

Alex Robertson, director of communications at the Electoral Commission, said: "No-one should miss out on voting in this historic referendum because of the problem with the Government's registration website last night.

"We said this morning that legislation should be introduced to extend the registration deadline and we're pleased the Government will now be making this change.

"We are urging everyone who is not already registered to vote to take this last chance to do so before the end of Thursday."

Labour and Liberal Democrats have promised their support in rushing emergency legislation through Parliament.

But Lib Dem leader Tim Farron branded the situation a "shambles" and said some voters would inevitably have given up the effort to register because the Government had been "incredibly slow" to act.

Pro-Brexit Tory Sir Gerald Howarth said voters had only themselves to blame if they missed out after leaving registration to the last minute, telling MPs: "P eople have had months and months in which to register and ... if they left it to the last minute and all tried to register yesterday, that's their fault ...

"We should not change our regulations in the middle of a very important referendum campaign simply to suit those who haven't organised their personal affairs well enough to secure their registration in good time."

Backing the planned extension, Mr Gove added: "People will only have one chance to vote on whether they share free movement of people with Turkey, so the more people who register to vote on June 23, the better, and we welcome the extension of the registration deadline.

"It is particularly important given how few young people normally vote and I hope that this election will be different."

The extension will not apply to Northern Ireland as non-digital registration methods were used there.